Cut layer, about a yard — how much do you want? (3,4)
I believe the answer is:
'how much do you want?' is the definition.
(phrase used when pouring a drink)
'cut layer about a yard' is the wordplay.
'cut' becomes 'saw' (sawing is a kind of cutting).
'layer' becomes 'hen' (I've seen this in another clue).
'about' is an insertion indicator.
'a yard' becomes 'y' (abbreviation).
'sawhen' placed around 'y' is 'SAY WHEN'.
(Other definitions for say when that I've seen before include "Pourer's words", "Request to be told to stop pouring drink", "Words from the pourer?", "Tell me when to stop pouring".)